In recent years, a reduction in power consumption of electronic equipment has been desired and the voltage for operating a transistor mounted in an integrated circuit is controlled precisely for each kind of circuit. An integrated circuit has a circuit portion that operates on a base voltage that is supplied from the outside and a circuit portion that operates on a voltage other than the base voltage. The voltage other than the base voltage is generated from the base voltage by using a charge pump circuit or the like, or from the base voltage or a power source voltage that is generated separately by using a low drop out circuit. The power source circuit of the integrated circuit such as this is called an adaptive supply voltage (ASV) system.
Further, in order to reduce the power consumption of an integrated circuit, it is effective to reduce the leak current of a transistor that is mounted in the integrated circuit. As one of method for reducing the leak current of a transistor, an adapting body bias (ABB) system that controls the back gate potential of a transistor is known. The back gate voltage that controls the back gate potential of a transistor is generated by, for example, a low drop out circuit because the current-carrying capacity is small.
In the case where a low drop out circuit that implements the ABB system is provided in an integrated circuit adopting the above-described ASV system, the back gate voltage that is higher than the base voltage is generated from the base voltage whose power source supply capacity is high while the back gate voltage is equal to or less than the base voltage. Specifically, a capacitive element that holds the back gate voltage is charged up to the back gate voltage by the low drop out circuit after being changed up to the base voltage by the base power source. In the low drop out circuit, for example, a transistor is connected between the terminal to which the high-voltage power source voltage is supplied and the terminal from which the back gate voltage is output, and the turning-on/off of the transistor is controlled in accordance with the results of comparison between the back gate voltage and a reference potential.
As described previously, in the power source sequence in the ASV system, the high-voltage power source voltage is generated by the charge pump or the like, and therefore, the supply of the high-voltage power source voltage to each unit within the integrated circuit is delayed from the supply of the base voltage. Consequently, the supply of the high-voltage power source voltage to the low drop out circuit is delayed from the supply of the base voltage. Due to this, in the low drop out circuit, the base voltage is applied to the terminal from which the back gate voltage is output before the high-voltage power source voltage is supplied, and therefore, a current flows backward.
In order to prevent such a backflow of a current in the low drop out circuit, a transistor is diode-connected between the transistor of the low drop out circuit and the supply terminal of the high-voltage power source voltage.